Graft-versus-host disease (GvHD) is the major limiting factor to the widespread use of allogeneic stem cell transplantation. GvHD is due to the recognition by donor T cells of minor or major MHC disparities found on antigen-presenting cells in the host, which activate donor T cells via a presumed Th1/Tc1 pathway. If this model were true, approaches that block the function or activity of Th1/Tc1 T cells would diminish the incidence or severity of GvHD. Interestingly, experiments performed over the past 15 years have not found this. Multiple studies have shown that blocking the activity of IFN- after allogeneic transplant enhanced the incidence and severity of GvHD. Th2 cells have not been shown to mediate lethal GvHD in multiple different murine models suggesting that an unidentified population of T cells could be responsible for acute GvHD. Several years ago multiple different investigators found a new subset of T cells that generate the cytokines IL- 17A, IL-17F, IL-21, IL-22, and TNF- . These Th17 CD4+ T cells generate proteins that recruit neutrophils and are generated via activation of the transcription factors ROR t and ROR . Th17 cells have been implicated in the pathogenesis of multiple different autoimmune diseases including experimental autoimmune encephalitis, collagen-induced arthritis, and inflammatory bowel disease. Th17 cells were found to compose a small population of T cells that could mediate autoreactivity in a chronic GvHD model. Our collaborators at Wyeth found that Th17 cells mediated extensive skin disease in a transplantation model for psoriasis and that blocking the function of IL-22 produced by Th17 cells greatly diminished disease severity. Our group has recently demonstrated that Th17 cells can mediate significant skin, liver, lung and GI tract pathology in murine GvHD models. Quite recently as shown in our preliminary data, we found that (1) the absence of the Th17 transcription factor ROR t in donor T cells renders them unable to generate GvHD in two clinically relevant transplantation models (2) that skin biopsies from patients with acute and chronic GvHD express significant quantities of Th17 cytokines and (3) that Th17 cells generate significant quantities of IFN- in the lymphopenic post transplant environment. These findings have led us to hypothesize that the Th17 program is critical to the generation of acute GvHD and that these T cells may (1) have a specific tropism for the skin, lung and GI tract and (2) generate much of the IFN- produced post stem cell transplantation. The current proposal extends our novel findings regarding the role of Th17 cells in GvHD by determining (1) the Th1/Th17 cytokine profile and function of Th17 cells tracked longitudinally after transplantation from ROR t- GFP mice (2) the specific role that the Th17 cytokine IL-22 plays in cutaneous pathology from GvHD and (3) whether blocking the migration of Th17 cells to the skin and lung will prevent GvHD at these sites. This proposal has the potential of radically altering our understanding of the biology of acute GvHD and offering new avenues for its prevention and treatment. PUBLIC HEALTH RELEVANCE: Graft-versus-host disease (GVHD) is a significant complication of allogeneic stem cell transplantation (allo-SCT) mediated by donor T cells that recognize minor and major histocompatibility complex (MHC) differences in the host. Previous work had suggested that acute GVHD was driven through a Th1/Tc1 pathway due to donor T cells generating IFN- . However, blocking IFN- paradoxically exacerbated GVHD in animal models suggesting that another pathway could mediate acute GVHD. We have found that T cells polarized toward a Th17 phenotype cause significant GVHD involving the GI tract, liver, lungs and skin in murine models. Additionally, we have found that these Th17 cells can generate significant quantities of IFN- suggesting that the production of this cytokine may not be generated solely by Th1 cells. In the current proposal, we will evaluate the function of Th17 cells in GVHD models and determine how they contribute to the production of IFN- . Cytokines produced by Th17 cells that are implicated in cutaneous disease in other models will be evaluated and methods to block the migration of Th17 cells investigated as a novel therapeutic approach. This work has the capacity to fundamentally alter our perceptions of the biology of GVHD and offer new avenues for therapy.